The operation of an optical head that is one of conventional optical information processors is described with reference to FIGS. 18(a) and 18(b). Light emitted from a semiconductor laser 18-1, an exemplary light source, passes through a hologram 18-5 as a separation element and then is focused on an optical disk 18-2, as an exemplary information recording media, by an objective lens 18-3. After passing through the objective lens, light reflected from the optical disk is diffracted by the hologram and is incident onto first photodetectors 18-4-1 and 18-4-2. An aperture in an optical path from the light source to the optical disk (hereinafter referred to simply as an “incoming path”) through which light passes is determined by an objective lens holder 18-6. A circular aperture is used in many cases. An aperture NA corresponds to a diameter of light being incident onto the objective lens. The diameter D satisfies the relationship ofD=2×f×NA, wherein f represents a focal length of the objective lens. Since the focal length f is constant, the size of the NA corresponds to the size of the diameter D. The aperture in an optical path from the optical disk to the photodetectors (hereinafter referred to simply as a “return path”) through which light reflected from the optical disk passes also is determined by the objective lens holder 18-6. Therefore, the apertures in the incoming and return paths are equal.
A detection case of various signals is described. When the hologram is formed of a part of a Fresnel lens, it can be formed so that diffracted light in one side is focused before reaching the photodetector 18-4-1 and diffracted light in the other side is focused at a position behind the photodetector 18-4-2 as shown in FIG. 18. As shown in a view seen from the A direction in FIG. 18, when the respective photodetectors 18-4-1 and 18-4-2 are formed while being divided into three parts, a focus error signal FE in a SSD (spot size detection) system can be detected from the calculation result of outputs from the respective photodetectors. The FE can be obtained from either:FE=(18-4-1b)−(18-4-2b)  (1) or FE=((18-4-1a)+(18-4-1c)+(18-4-2b))−((18-4-1b)+(18-4-2a)+(18-4-2c))  (2). 
When a track direction on an optical disk coincides with the information track direction shown in FIGS. 18(a) and 18(b), a far field pattern as a diffraction pattern produced by a track is formed at spots on the photodetectors as shown in the view seen from the A direction. Therefore, the tracking error signal TE can be obtained from any one of:TE=(18-4-1a)−(18-4-1c)  (3), TE=(18-4-2a)−(18-4-2c)  (4), and TE=((18-4-1a)+(18-4-2c))−((18-4-1c)+(18-4-2a))  (5). 
A data information signal RF of an optical disk can be obtained from all the outputs of the photodetector 18-4-1 or 18-4-2, or the total outputs of the photodetectors 18-4-1 and 18-4-2.
FIG. 19 shows a configuration of an optical disk device in another conventional example using two laser beam sources that emit beams with different wavelengths from each other. This optical disk device has two laser beam sources 19-1 (emitting a beam with a wavelength λ1) and 19-2 (emitting a beam with a wavelength λ2) that emit beams with different wavelengths from each other. The laser beam 19-21 with a wavelength λ1 (in the case of DVD or the like, λ1=660 nm) emitted from the laser beam source 19-1 passes through a polarization hologram element 19-3.
This polarization hologram element is formed by forming a grating with a depth of d in a substrate made of an anisotropic material such as lithium niobate and filling groove parts of the grating with an isotropic material (with a refractive index of n1). Generally, given the phase difference φ between a beam passing through a groove portion and a beam passing between the groove portions, transmittance is represented by cos2(φ/2). When the substrate has refractive indexes of n1 and n2 with respect to polarized lights parallel and perpendicular to the grating grooves respectively, φ=0 holds with respect to the polarized light parallel to the grating grooves and therefore the transmittance is 1. On the other hand, with respect to the polarized light perpendicular to the grating grooves, φ=2π(n1−n2) d/λ. Therefore, when the depth d is set to obtain φ=π, the transmittance is 0, i.e. the polarized light is totally diffracted.
Consequently, when considering the polarization direction of the beam 19-21 emitted from the laser beam source 19-1 and groove orientation of the polarization hologram element 19-3, the laser beam 19-21 is allowed to pass through the element 19-3 without being diffracted. The transmitted light 19-22 is converted from linearly polarized light (S-wave) into circularly polarized light 19-23 by a ¼ wave plate 19-4, is reflected by a surface of a prism 19-5, and then is collimated into parallel light 19-24 by a collimator lens 19-6. The parallel light 19-24 enters an objective lens 19-8 mounted on a moving element 19-14 of an actuator via a mirror 19-7 for bending an optical path and is incident onto a signal surface 19-9 of the optical disk.
In the case of recording on the signal surface, by increasing the power for emitting beams of the laser beam source 19-1 and modulating light corresponding to a recording signal, a required signal is recorded on the signal surface 19-9.
The light 19-25 reflected from the signal surface 19-9 travels in the opposite direction to the incoming path. The light 19-25 is converted to linearly polarized (P-wave) light 19-26 by the ¼ wave plate 19-4 and passes through the polarization hologram element 19-3. In this case, due to polarization dependability of the element 19-3 the light is branched into a positive first-order diffracted light 19-27 and a negative first-order diffracted light 19-28 whose symmetry axis is the incident-light axis. The lights 19-27 and 19-28 are incident onto detection surfaces on photodetectors 19-10 provided adjacently to the laser beam source 19-1. Thus, a control signal and a reproduction signal are obtained to reproduce information.
On the other hand, a laser beam 19-29 emitted from the semiconductor laser beam source 19-2 emitting a beam with the other wavelength λ2 (in the case of CD or the like, 790 nm) passes through a hologram element 19-11 to be diffracted and branched into three beams (a positive first-order diffracted light, a negative first-order diffracted light, a zeroth-order light). The three beams pass through the prism 19-5 while being limited by an aperture element 19-12 provided on a light-incident surface of the prism 19-5 and are collimated by the collimator lens 19-6 into convergent light 19-30. Then, the convergent light passes through the objective lens 8 via a mirror 19-7 for bending an optical path, thus being incident onto a signal surface 19-15 of an optical disk whose substrate has a different thickness from that when using the laser beam source 19-1. In this case, the diffracted light caused by the hologram element 19-11 is allocated to three spots on the signal surface and is used for the detection of a tracking control signal and a reproduction signal by a so-called three-beam tracking method. Light 19-31 reflected from the signal surface 19-15 is diffracted by the hologram element 19-11 via the mirror 19-7, the collimator lens 19-6, and the prism 19-5. Then, the diffracted light is incident onto detection surfaces of photodetectors 19-16, thus detecting signals to reproduce information. The objective lens 19-8 is designed to have a shape that enables aberration to be minimum by optimally designing the aperture and the optical system for respective disks having a substrate thickness of 0.6 mm for the beam with the wavelength λ1 and having a substrate thickness of 1.2 mm for the beam with the wavelength λ2. In other words, with respect to the beam with the wavelength λ2, the aperture is limited by the aperture element 19-12 to form an optimum aperture.
With increase in density of the data information, further improvement in recording and reproducing ability is required in optical disk devices. Generally, in order to record and reproduce signals with higher density, a focusing spot on a disk is reduced in size. That is, it is conceivable that the wavelength of light emitted from a light source is shortened or NA of an objective lens is increased. However, in general-purpose optical disk devices used in a general office or at home, an available short-wavelength light source is a semiconductor laser emitting a red beam with 660 nm at present. A semiconductor laser emitting a beam with a shorter wavelength than that lacks in reliability and therefore it is difficult to use it for recording purpose in the present situation. When the NA of the objective lens is increased (i.e. when the aperture of an objective-lens holder is enlarged), recording/reproducing characteristics are improved in part. However, margins for tilt and defocus are reduced greatly, which has been a problem. In one or more embodiments, it is a first object of the present invention to provide an optical disk device in which excellent recording and reproduction can be performed on an optical disk with higher density and the margins are not reduced at the same time.
On the other hand, there have been the following three problems in a conventional optical disk device using the two laser beam sources shown in FIG. 19.
Firstly, when the lens is shifted in a track direction of an optical disk, the relative position of the lens and the aperture element varies, thus causing asymmetry in the aperture. Consequently, aberration (mainly spherical aberration and coma aberration) is increased, thus deteriorating signal quality considerably.
Secondly, similarly when the lens is shifted, the relative position of the lens and the hologram varies and therefore unbalance in quantity of lights divided by the hologram and distributed to photodetectors occurs, thus causing offset of a signal due to DC components, which is not preferable in tracking control.
Thirdly, generally due to refractive index variance of an objective lens or a collimator lens, when the wavelength mode of a beam emitted from a laser beam source is changed by power modulation for recording and reproduction, momentary axial aberration (i.e. chromatic aberration) occurs. Consequently, a relative position error (defocus) between a lens and a signal surface is caused. In order to prevent this, any chromatic aberration compensation element is required. FIGS. 20(a) and 20(b) show a cross-sectional structural view and a plane view of a conventionally proposed chromatic-aberration compensation element 20-160 (see JP-A-6-82725 about the details). The chromatic-aberration compensation element 20-160 is formed of a glass plate having a refractive index n in which a concentric stepped structure 20-150 is formed. In the figure, the phase of a beam with a wavelength λ that passes through the concentric stepped structural portion having a stepped depth t represented by:t=λ/(n−1) is shifted for 2π between adjacent stepped portions. However, with respect to undulation, the same wavefront is formed. On the other hand, when the wavelength is shifted from λ, the phase of the light is shifted slightly between adjacent stepped portions. However, since this stepped structure is formed in a concentric shape, an almost spherical wave is generated in a direction canceling axial aberration caused by the chromatic aberration. Thus, the aberration can be compensated by combining this element with a lens.
In order to solve all the three problems of the optical disk device shown in FIG. 19, it is preferable to mount all the components described above (the aperture element, the hologram element, the chromatic-aberration compensation plate) on a moving element. However, when all these elements are mounted, the moving element becomes very heavy and in addition, it is difficult to keep the actuator in balance. Further, as the weight of the actuator increases, more energy is required for its operation, thus causing problem of high power consumption. In addition, since all the elements must be positioned accurately with respect to the center of lenses (the center of optical axes), highly accurate assembly processes are necessary, thus decreasing mass-productiveness. In one or more embodiments, a second object of the present invention is directed to solve these problems.